Lens sheets, which include prism sheets used as backlight units for liquid crystal display devices and the like, Fresnel sheets and lenticular lens sheets used as projection screens for projection televisions and microfilm readers, and lenticular lens sheets used for stereoscopic photography and the like, have lens sections which are formed using active energy ray-curing compositions. Such lens sheets are constructed, for example, with a lens section made of a cured active energy ray-curable composition and formed on a transparent substrate. The active energy ray-curable composition used to form the lens section must have a variety of properties, including adhesion to the transparent substrate, releasability from the lens mold, and suitable optical characteristics.
For example, in the case of backlights used for the color liquid crystal panels in color liquid crystal display devices for notebook computers, and in liquid crystal televisions and video-integrated liquid crystal televisions, it is important to reduce the power consumption to a minimum without lowering the brightness of the backlighting, and therefore greater optical efficiency has been desired for backlights. In this regard there has been proposed a backlight with enhanced brightness, wherein a prism sheet forming a prism row 2 on one side as shown in FIG. 1(a) is mounted on the light-emitting surface 4 of the backlight 3 shown in FIG. 1(b) in the manner shown in FIG. 1(c).
The proposed prism sheet is usually one with a prism row formed by press working on one side of a transparent sheet made of a thermoplastic resin, or one with a prism row, made of an ultraviolet ray-curable composition, formed on one side of a transparent sheet, with the material for the former being polymethyl methacrylate (refractive index: 1.49) or polycarbonate (refractive index: 1.59) and the ultraviolet curing composition used for the latter being a (meth)acrylate-based composition (refractive index of cured resin: 1.49-1.55).
However, with methods of fabricating prism sheets by the pressure formation method using thermoplastic resin transparent sheets, it is difficult to achieve a balance between the refractive index and transparency of the materials to make a prism sheet with a satisfactory brightness-enhancing effect, and with methods of fabricating prism sheets using the latter ultraviolet curing (meth)acrylate-based compositions, it becomes more difficult to sufficiently increase the refractive index the more the brightness of the prism sheet is enhanced. Furthermore, although prism sheets fabricated using materials with high refractive indexes contribute greatly to increased front brightness of the backlight, a disadvantage has been that the light transmittance tends to be lowered.